Surface treatment for micromachining

ABSTRACT

A fine surface treatment for micromachining having an etching speed whose difference is smaller to oxide films each obtained by a different method as well as conditions of forming film or having different concentration of various impurities such as P, B and As in the film, and also having a practical etching speed to each of the films. The surface treatment for micromachining contains 0.1 to 8 weight percent of hydrogen fluoride and not less than 40 weight percent to not more than 47 weight percent of ammonium fluoride. It should be noted that it is preferable the surface treatment agent contains 0.001 to 1 weight percent of surfactant.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface treatment for micromachiningand, more particularly, to a surface treatment used for fine treatmentof silicon oxide in the manufacturing process of semiconductor devices.

2. Description of the Related Art

Fine treatment of film such as oxide film on a wafer surface is one ofthe most important processes in the manufacturing process ofsemiconductor integrated circuits. Its importance has been increasinglygrown in association with progress of high integration of circuits.

Conventionally, a mixed solution (buffered hydrogen fluoride) ofhydrogen fluoride (HF) and ammonium fluoride (NH₄ F) is applied for finetreatment of film such as oxide film, and is usually prepared by mixing40 percent NH₄ F solution and 50 percent HF solution in various mixingrations such as in a range of 400:1 to 6:1 so as to control etchingspeed of silicon oxide film from 2.7 nm/min to 120 nm/min.

There are some cases in the manufacturing process of semiconductors thatoxide films having various properties are formed depending on eachmethod of forming film as well as on conditions thereof and that thosefilms exist on the same substrate surface. When oxide films having adifferent property are subjected to etching with the buffered hydrogenfluoride, for example, when an SiO₂ film and an SiO₂ film containing Pare subjected to etching with the buffered hydrogen fluoride, a step isformed at the boundary of the films because of different etching speedof the films.

Although the step of around 50 nm hardly affects the following processin the case of 1 μm design rule semiconductor process, it is clear thatin the manufacturing process of submicron semiconductor integratedcircuits even the step of around 20 nm causes some trouble to occur inthe manufacturing process and that circuit yields are reduced. Namely,it is understood that the step formed in etching process may becomelarger than from a depth of focus of photolithographic optical system,which induces reduction of yields in the patterning process. Then, it isalso clear that this problem will be larger in association with furthertechnological progress to reach a manufacturing process ofquarter-micron semiconductor integrated circuits.

An object of the present invention is to provide, in the light of thecircumstances described above, a fine surface treatment formicromachining which has nearly the same etching speed for various oxidefilms, each obtained by a different method as well as conditions offorming film or having different concentration of various impuritiessuch as P, B, and As in the film and which also has a practical etchingspeed to each of the films.

SUMMARY OF THE INVENTION

According to the present invention a surface treatment formicromachining contains 0.1 to 8 weight percent of hydrogen fluoride andmore than 40 weight percent to not more than 47 weight percent ofammonium fluoride.

A surface treatment for micromachining according to the presentinvention is prepared by dissolving ammonium gas into a solution ofhydrogen fluoride.

A surface treatment for micromachining according to the presentinvention contains 0.001 to 1 weight percent of surfactant.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become more apparent and theinvention will be better understood by reference to the followingdescription of an embodiment of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a schematic diagram showing a DRAM manufacturing process inEmbodiment 5; and

FIG. 2 is a view conceptional showing an example of a result ofmeasuring the surface of a silicon wafer in Embodiment 7 by an atomicforce microscope.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplification set out hereinillustrates one preferred embodiment of the invention, in one form, andsuch exemplification is not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

Buffered hydrogen fluoride based on the conventional technology has beenprepared by mixing a 40 percent NH₄ F solution and a 50 percent HFsolution. Accordingly, when hydrogen fluoride concentration iscontrolled, for example, in a range from 0.1 to 8 weight percent, theNH₄ F concentration in the solution is in a range from 33.6 to 39.9percent or less than the value.

On the other hand, the surface treatment for micormachining according tothe present invention comprises a mixed solution containing 0.1 to 8weight percent of hydrogen fluoride and more than 40 weight percent tonot more than 47 weight percent of ammonium fluoride (NH₄ F). It isfurther preferable that ammonium fluoride is not more than 45 weightpercent. With the range of those concentration, it is possible tosuppress a ratio of etching from more than 1.1 based on the conventionaltechnology to a range from 1.03 to 1.07 for various types of oxide filmsirrespective of forming method (for example, thermal oxidation and CVD),thermal treatment condition, or the amount of impurity such as P, As andB in the film.

Namely, a step between oxide films after being subjected to etching ispossibly suppressed within the depth of focus in a submicron stepperoptical system.

It should be noted that the concentration of hydrogen fluoride iscontrolled from 0.1 to 8 weight percent in the invention because etchingspeed becomes 1 nm/min or less for the concentration lower than 0.1percent and crystal precipitation temperature becomes 35° C. for theconcentration higher than 8 percent.

It is preferable that surfactant of 0.001 to 1 weight percent iscontained in the surface treatment for micromachining. If a spacebetween resists is around 0.5 μm or less, uniformity in etching to theoxide film is reduced because the our face of oxide films will hardlyget wet to surface treatment for fine surface treatment. However,wettability to the resist surface is improved by adding the surfactantto the surface treatment and, as a result, etching uniformity to thesilicon oxide film is further improved. If the Si surface is exposed,the roughness of the surface can be suppressed by the surfactant, andfor this reason a highly performable device can be realized. The effectof addition of the surfactant thereto is hardly recognized if theconcentration is less than 0.001, and the same effect will no longerchange for the concentration is higher than 1 percent or more.

Aliphatic amine (C_(n) H_(2n+1) NH₂ ; n=7 to 14), aliphatic carboxylicacid (C_(n) H_(2n+1) COOH ; n=5 to 11) and aliphatic alcohol (C_(n)H_(2n+1) OH; n=6 to 12) are preferably used as a surfactant of thepresent invention. It is especially preferable to combine at least twotypes of the three surfactants which are selected for utilizationaccording to NH₄ F concentration and HF concentration therein.

The surface treatment for micromachining according to the presentinvention is prepared as follows.

Namely, the surface treatment can be produced by a method of dissolvinghigh-purity NH₄ F powder into a HF solution, a method of mixing ahigh-purity NH₄ OH aqueous solution in high concentration produced byabsorbing NH₃ gas in ultrapure water and a 50 percent HF solution, and amethod of absorbing NH₃ gas in a HF aqueous solution (there may be amethod of producing NH₄ F in high concentration by absorbing NH₃ gas ina HF solution and mixing the produced NH₄ F and 50 percent HF in adesired ratio) or the like. The method of absorbing NH₃ gas in a HFaqueous solution is the most preferable of all because a higher puritysolution can be obtained.

However, when NH₄ F concentration is more than the saturatedconcentration, crystal precipitation occurs. Once the precipitation hasoccurred, it is hard to dissolve the crystal and return to the uniformsolution just by increasing temperature by several degrees. Thisprecipitation should be removed to prevent of nonuniformity etching dueto particles.

It should be noted that even if the crystallization has occurred it ispossible to prevent particles of crystallization from being adhered tothe wafer surface by adding the surfactant to the surface treatment.

When a surface treatment for micromachining is actually used insemiconductor production facilities, the surface treatment formicromachining is introduced from a large-sized tank or a medium-sizedvessel of around 100 L to a processing bath through a PFA tube or thelike. Also, semiconductor production facilities generally maintain aroom temperature at a range from 20 to 23° C., and for this reason it isrequired to control the concentration so that precipitation does notoccur at this temperature. Namely, the concentration is required to benot more than 45 percent. If the precipitation temperature of aprocessing liquid is 18° C., the temperature in the tank and themedium-sized vessel or the tube is required to be not less than 18° C.

The surface treatment for micromachining according to the presentinvention is used for film such as silicon oxide film, nitride film, andoxide films of polysilicon, Al, Ta, or Ti. As silicon oxide film, thereare included film such as thermal oxide film (including dry oxide filmand wet oxide film which contains one to eight weight percent of arsenicand phosphorus introduced by means of ion implantation or the like), CVDfilm (also including film which contains one to eight weight percent ofboron, arsenic, and phosphorus), and TEOS film (including film whichcontains one to eight weight percent of phosphorus and boron).

Embodiment 1

In one form of the invention thereof, an eight-inch silicon wafer wassubjected to wet-oxidation for 120 min at 1000° C. to form thermal oxidefilm with a thickness of 400 nm.

Then, after resist film was obtained, the wafer has formed with apattern, with each pattern whose one side was 1 mm formed to checkers,1×10¹⁶ /cm² Ion of As implanted to 40 KeV. Then the wafer was subjectedto heat treatment for 60 min. at 800° C., and a layer with a thicknessof 200 nm including two weight percent of As was formed thereon.

The two types of thermal oxide film formed as described above weresubjected to etching at 25° C. by using a solution of 1.0 percent HFconcentration and with NH₄ F concentration in various degrees. Table 1shows the result. It should be noted that etching speed of theAs-contained thermal oxide film as well as of the thermal oxide filmwere measured through measurement of film thickness of the oxide films.

                  TABLE 1                                                         ______________________________________                                                                 Etching                                                  rate ratio                                                                    As-                                                                           contained                                                                           Etching rate (nm/min)                                                                        thermal                                                        As-                oxide  Crystal                                     NH.sub.4 F contained Thermal film/ precipitate                                Concentration thermal oxide Thermal temperature                               (%) oxide film film oxide film (° C.)                                ______________________________________                                        10.0      29.6     22.1      1.338  -7                                          30.0 25.8 22.9 1.130 -15                                                      40.1 17.7 16.8 1.056 -20                                                      42.0 16.0 15.2 1.050 0                                                        44.1 14.5 13.9 1.040 15                                                       45.0 14.2 13.7 1.039 18                                                       45.2 13.9 13.4 1.037 21                                                       46.0 13.0 12.5 1.041 24.9                                                     47.0 24.0 22.8 1.053 29                                                     ______________________________________                                    

As clearly understood from Table 1, when the NH₄ F concentration was40.1 percent or more, etching speed ratio between the thermal oxidefilms was less than 1.06, and a step formed in the process was less than11.4 nm when the As-contained thermal oxide film had completely beenremoved. The step was found to give no effect to the process ofsubmicron fine surface treatment. On the other hand, in theconcentration based on the conventional technology (30 percent), theetching speed ration was 1.13, and the step was 23 nm.

The etching speed ratio becomes closer to 1.0 as the NH₄ F concentrationis increased, which is more preferable, but precipitation easily occursin 46 percent because the precipitation temperature is 24.9° C., and forthis reason the temperature has to be controlled with higher precision.It should be noted that occurrence of crystallization generatesnonuniformity in etching or the like, which turns out to be some troubleto finer patterns.

With the experiments performed at the etching temperature of 35° C. andusing a solution containing NH₄ F concentration of 47 percent, it hasbeen found that etching speed ratio was 1.05 and the same etching couldbe performed without the appearance of a step.

It should be noted that an etching solution at a room temperature ispreferable used because there will be larger possibility of elution ofimpurity from a vessel or the like if the temperature in the solution isincreased.

Embodiment 2

In another embodiment, the same evaluation was made by etching with asolution in variations of HF concentration to a silicon wafer by usingoxide film formed thereon in the same manner as that of Example 1. Itshould be noted that NH₄ F concentration was 44.1 percent. Table 2 showsthe result.

                  TABLE 2                                                         ______________________________________                                                                 Etching                                                  rate ratio                                                                    As-                                                                           contained                                                                           Etching rate (nm/min)                                                                        thermal                                                        As-                oxide  Crystal                                     NH.sub.4 F contained Thermal film/ precipitate                                Concentration thermal oxide Thermal temperature                               (%) oxide film film oxide film (° C.)                                ______________________________________                                        0.1       25       24        1.041  13                                          0.4 62 60 1.038 17                                                            1.0 145 139 1.040 15                                                          1.6 220 210 1.048 13                                                          2.0 267 253 1.053 10                                                          3.0 321 305 1.051 15                                                          8.0 728 688 1.058 24                                                        ______________________________________                                    

As clearly understood from Table 2, and etching rate can be increasedwhile a ratio of etching speed between the films is kept to besuppressed at a lower level as it is even if HF concentration isincrease, and etching can be performed with high precision to varioustypes of film thickness. It was found from the precipitation temperaturethat the HF concentration is preferable to be not less than 0.1 percent.It was also found that the temperature was less than 24° C. with theconcentration of not more than eight percent, and the temperature wasless than 15° C. with the concentration of not more than three percent.

Embodiment 3

In another form of the invention thereof, a resist film was formed on aneight-inch silicon wafer with the oxide film in Example 1 formedthereon, a resist pattern of 0.5 μm×0.5 μm was open by being exposed anddeveloped. As a result, this wafer was subjected to etching with anetching solution containing 1.0 percent HF and 42.0 percent NH₄ F andwith an etching solution further containing C₈ H₁₇ NH₂ (100 ppm) and C₇H₁₅ COOH (100 ppm), nonuniformity of etching within the silicon waferwas three percent with the solution containing surfactant, which made itclear that the uniformity could be higher as compared to the solutionwith no surfactant included (10 percent).

Embodiment 4

In yet another form of the invention thereof, non-doped SiO₂ film andSiO₂ (BPSG) film in which B and P had been doped were formed on asilicon wafer by a thermal CVD method, and etching speed rations weremeasured using etching solutions having various types of composition.Table 3 shows the result. It should be noted that etching temperaturewas set to 25° C. Also, the conditions for forming film are as follows.

Non-doped SiO₂ film:

Gas: SiH₄ :N₂ O=1:10

Pressure: 0.02 atm

Temperature: 600° C.

BPSG film:

Gas: SiH₄ :O₂ :B₂ H₆ :PH₃ =10:1000:2:1

Pressure: 1.2 atm

Temperature: 400° C.

                  TABLE 3                                                         ______________________________________                                                                           Ratio                                           between                                                                       etching                                                                    NH.sub.4 F HF Etching rate (nm/min) rates                                   Concentration                                                                          Concentration       Non-doped                                                                             BPSG/Non-                                  (%) (%) BPSG film doped                                                     ______________________________________                                        5.0      1.0        56.7     19.8    2.864                                      10.0 1.0 50.0 22.1 2.263                                                      15.0 1.0 31.2 22.1 1.415                                                      20.0 1.0 29.0 13.8 1.218                                                      20.0 2.0 59.0 50.1 1.178                                                      20.0 4.0 98.9 82.3 1.202                                                      30.0 1.0 25.5 22.9 1.116                                                      30.0 2.0 53.6 47.8 1.121                                                      40.0 1.0 18.2 16.8 1.083                                                      40.0 2.0 32.9 30.2 1.089                                                      42.0 1.0 15.9 15.2 1.046                                                      42.0 2.0 31.2 29.5 1.058                                                      43.0 2.0 29.6 28.3 1.046                                                      44.0 1.0 14.2 13.7 1.036                                                      44.0 2.0 28.6 27.3 1.048                                                      45.0 1.0 13.0 12.6 1.032                                                      45.0 2.0 26.3 25.2 1.044                                                    ______________________________________                                    

As clearly understood from Table 3, it has been found that the etchingspeed ration could be smaller if the NH₄ F concentration was controlledfor exceeding 40 percent even if the films were CVD film, thus a stepformed after the etching being suppressed.

Embodiment 5

In another form of the invention thereof, in the manufacturing processof DRAM memory cells, as shown in FIG. 1, a film of p-polysilicon, andthen non-doped CVD (SiO₂) film were formed. Then, BPSG film forsmoothing a surface was formed and subjected to thermal re-flow, andthen the surface of the film was smoothed by subjecting this BPSG filmto etch-back.

The BPSG film and the non-doped SiO₂ film were formed under the sameconditions as described in Example 4. Also, the re-flow was carried outin a nitrogen gaseous atmosphere at the temperature of 900° C.

The etching was performed, so as to leave the non-doped SiO₂ film by atleast 50 nm, with an etching solution I (containing 44 percent NH₄ F and1.0 percent HF) as well as with an etching solution II (containing 30percent NH₄ F and 1.0 percent HF) at 25° C.

As a result of observation on the surface after the etching, it wasfound that the step formed on the surface was 7 nm when the etchingsolution I was used and was 20 nm when the etching solution II was used.

Embodiment 6

In another form of the invention thereof, effects due to addition ofsurfactant to the solution was studied.

At first, as an etching solution, a solution of 43 percent NH₄ F and 2.0percent HF and a solution obtained by further adding the same surfactantas described in Example 3 to this solution were left at the temperatureof (-5° C.), lower than the precipitation temperature (2° C.).

The number of particles whose diameter is not less than 0.2 μm wasmeasured in the solution before and after the solutions were left byusing a particle measuring device based on a laser diffusion system(KL-22 produced by RION).

A six-inch silicon wafer was taken out, after soaking in the varioustypes of solutions for five minutes and the number of particles on thewafer surface was measured by using a wafer-surface foreign mattermeasuring device based on a laser diffusion system (Surfscan 6200produced by TENCOL). Table 4 shows the result of measuring the number ofparticles in the solution as well as on the wafer surface.

                  TABLE 4                                                         ______________________________________                                                                          The number                                      The number of particles                                                       of submerged on the wafer                                                     particles surface                                                           Chemical  (unit/ml) (unit/wafer)                                            ______________________________________                                        Surfactant/                                                                             Before being left                                                                         50          48                                            Without After being left 2350 1200                                            Surfactant/With Before being left 53 15                                        After being left 2500 14                                                   ______________________________________                                    

The crystal precipitation increases when the temperature in the solutionis less than the precipitation temperature, and the precipitationinduces nonuniformity in etching or the like, but, as clearly understoodfrom Table 4, addition of the surfactant to the solution makes itpossible to suppress the number of particles to be adhered on the waferand to prevent nonuniformity in etching although particles in thesolution can not be reduced.

Namely, even if a large number of crystallizations exist in thesolution, those particles are not adhered on the wafer surface, so thatit is possible to prevent a reduction of etching yields. Accordingly,any etching solution can easily be managed.

Embodiment 7

In another form of the invention thereof, resist film (1 μm) was formedon an eight-inch silicon wafer with 1 μm-thermal oxide film formedthereon, and 2,000 units of pattern for each of various-sized contactholes were formed on the oxide film.

Then, the thermal oxide film was etched by about 100 nm for four min at25°C. by using an etching solution containing 45 percent NH₄ F and 2.0percent HF or an etching solution obtained by adding the surfactant (200ppm in total) in Example 3 to the etching solution described above.

After the etching, the resist was peeled with a mixed solution ofsulfuric acid and hydrogen peroxide, and then the surface was inspectedby the atomic force microscope (Nano Spec III produced by DigitalInstruments).

FIG. 2 shows one of surface patterns measured by the atomic forcemicroscope, and Table 5 shows a relationship between the diameters ofcontact holes and defective rate of contact holes.

                  TABLE 5                                                         ______________________________________                                                      Diameter of contact                                                                        Defective contact                                    Etching solution holes holes                                                ______________________________________                                        Surfactant/Without                                                                          2.0          10                                                    1.0 20                                                                        0.5 45                                                                       Surfactant/With 2.0 0                                                          1.0 0                                                                         0.5 0                                                                      ______________________________________                                    

As shown in Table 5 as well as in FIG. 2, the addition of the surfactantto the solution makes it possible to completely eliminate defectivenessin etching even for a fine pattern.

Although it is also possible to soak the film once in ultrapure waterwith the surfactant added thereto and then to subject the soaked film toetching with an etching solution without adding directly the surfactantto the etching solution, some troubles such as incoming dusts andsolution of the etching solution easily occur, and for this reasonprevious addition of surfactant to an etching solution is preferable.

The surface treatment for micromachining according to the presentinvention, namely the surface treatment containing 0.1 to 8 weightpercent of hydrogen fluoride and more than 40 weight percent to not morethan 47 weight percent of ammonium fluoride can make a smallerdifference between each speed of etching to oxide films each obtained bya different method and conditions of forming film as compared with thoseof the etching solution based on the conventional technology.

As a result, a step on an oxide film generated in an etching process canbe suppressed, and for this reason submicron- and furtherquarter-micron-ultra fine surface treatment can also be carried out withhigh yields.

While this invention has been described as having a preferred design,the present invention can be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

What is claimed is:
 1. A surface treatment for micromachining thesurface treatment comprising 0.1 to 8 weight percent of hydrogenfluoride and more than 40 weight percent to not more than 47 weightpercent of ammonium fluoride.
 2. A surface treatment for micromachiningaccording to claim 1 prepared by dissolving ammonium gas into a solutionof hydrogen fluoride.
 3. A surface treatment of micromachining accordingto claim 1 containing 0.001 to 1 weight percent of surfactant.
 4. Asurface treatment of micromachining according to claim 2 containing0.001 to 1 weight percent of surfactant.
 5. A method for micromachining,comprising the step of:Applying a surface treatment containing 0.1 to 8weight percent of hydrogen fluoride and more than 40 weight percent tonot more than 47 weight percent of ammonium fluoride.
 6. The method ofclaim 5 further comprising the step:Preparing the surface treatment bydissolving ammonium gas in a solution of hydrogen fluoride.
 7. Themethod of claim 5 wherein the surface treatment further contains 0.001to 1 weight percent of the surfactant.
 8. The method of claim 6 whereinthe surface treatment contains 0.001 to 1 weight percent of thesurfactant.